Three phase gas-liquid-solid separation using centrifugal forces
Dr.-Ing. M. Creutz
Financed by the "Deutsche Forschungsgemeinschaft", SFB 264
In underwater thermal cutting processes of metal structures, such as underwater plasma arc cutting, gaseous and solid contaminants are emitted. The melted material of the joint remains as a suspension of metalloid particles with particle sizes ranging from some millimetres down to sub micron particles [1-5]. Plasma and cutting gases lead to high vortex flows and flotation of the small particles. The contaminants reduce the optic transparency of the water and thus have to be avoided in processes controlled by systems based on visual observation. Under water cutting processes are already being used for the demolition of nuclear power plants  and . Especially for that purpose, contaminants have to be completely removed. For the process of sucking off the contaminants and separating them into gas, clean water and slurry, a tool is developed that is shown in figure 1. The tool is intended to form an integral part of the plasma arc cutting system. Investigations on the exhaust of multiphase suspensions have been carried out . This text focuses on the separation only.
Due to its application in under water cutting processes there are a number of important demands regarding the design of a separator:
A three phase liquid-solid-gas suspension is to be separated simultaneously. The separator must be reasonable small. To obtain high separation forces, centrifugal forces are used.
The volumetric gas flow rate is approximately 10 times higher than the liquid flow rate. This ratio is technically uncommon for solid-liquid separation.The volumetric solid- fraction in the water is between 1 and 7 %. The multiphase flow in the inlet is dominated by the two phase gas-liquid flow.
Due to the unsteady sucking process and due to the dynamic behaviour of the multiphase flow, the flow in the separator is turbulent and transient.Typical flow patterns are slug and churn flow. The separator must be able to act as a slug catcher. To obtain a proper transportation of the solids from the separator to the water surface, the flow velocity and thus the pressure drop in the water pipeline has to be high. This can not be achieved by sucking the water from the water surface, thus the separator has to be a slurry pump.
The separator can be used in all applications, where two- or three-phase flow has to be separated under space limited conditions. For instance in thec hemical and in the off-shore industry it can be used as a combination of pump and separator.
In figure 2 the separator is shown schematically. It consists of a stationary casing and a rotating shaft with an impeller. In contrast to the impeller of a centrifugal pump, this impeller is open on both of its sides. The three phase flow enters the separator axially. By the centrifugal forces inside the impeller, liquid and solids are forced in the radial direction into a ring chamber. The gas flows through the impeller and leaves the separator. Inside the impeller, a rotating liquid level is formed. The position of this level is variable and has a large influence on the pressure in the ring chamber. The ring chamber consists of two parts divided by a cascade. Due to the centrifugal force of the rotating flow, large solid particles can not pass this cascade. Both of the ring chambers form a spiral and have two tangential outlets. A suspension of water, large particles and in a small amount of gas leaves the separator through the first outlet (outlet 1). Clean water with small size particles leaves the separator through the other outlet (outlet 2).
Some aspects of controlling the system In figure 3 the complete cleaning system including the sucking, the separation and transportation of under water contaminants is shown schematically. For each of the components the pressure of water and gas is shown. For simplicity, the flow of clean water is not recognised. Regarding the gas phase, no large pressure variations occur in the separator and in the pipeline. The difference between the hydrostatic pressure in the under water surrounding and the gas pressure in the separator is the driving force for the sucking process.
For a given pressure difference and gas flow rate, the liquid flow rate which is sucked off together with the gas is fixed. High liquid velocities are required to enable the slurry transportation. Thus, the frictional pressure drop in the suspension pipeline can not be neglected. This pressure drop depends on the liquid flow rate and has to be attained by the separator. For this reason, both, the back pressure and the liquid flow rate are imposed on the separator. In contrast to a centrifugal pump for which both parameters depend on each other, in the separator they are independent due to the variation in the liquid level. For this reason, the separator acts as a slug catcher.
Experimental investitgations have been done on the separation quality and the transient behaviour of the separator .
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